Synthesis, Hydrogen Uptake, And Catalysis Proterties Of Hierarchically Micro-and Mesoporous Metal-Organic Frameworks

Porous metal-organic frameworks (MOFs) are crystalline materials with pores and channels self-assembled by the bonding of metal ions with multifunctional organic ligands. They have attracted extensive attention recently due to interest in the creation of nano-sized spaces and their potential applications in gas storage, adsorption and separation, heterogeneous catalysis, molecular sensing, as well as magnetic materials. Compared with traditional porous carbons and inorganic zeolites, porous MOFs are synthesized under mild conditions and hold great promise for their ease of processability, flexibility, structural diversity, and geometrical control. However, to date porous MOFs are still largely restricted to the microporous regime despite negative impact of their small pore size on diffusion and mass transfer for their practical applications. As a result, to date reliable and reproducible methods to achieve robust mesoporous MOFs with tailored structures and tunable properties still remain a great challenge.In this dissertation, we explore a simple and versatile strategy that has allowed us to rationally design and synthesize for the first time hierarchically micro-and mesoporous MOFs with adjustable porosity. A novel supramolecular template strategy has been successfully applied to design mesostructured MOFs with tunable pore size, pore volume, and specific surface area. The main contents of this dissertation are as follows:1. Mesostructured MOFs with tailored porosity were obtained by choosing cetyltrimethyl ammonium bromide (CTAB) as supramolecular template. A series of hierarchically porous MOFs, i.e., [Cu3(BTC)2(H2O)3]n (HKUST-1), with adjustable interconnecting micropores and mesopores was prepared by self-assembly of the framework-building blocks, i.e., Cu2+ and benzene-1,3,5-tricarboxylate (BTC3-, in the presence of surfactant micelles. These mesostructured MOFs possess a mesopore system with tunable diameters from 3.8 up to 31 nm depending on different synthetic variables. Particularly, mesopore walls in these solids are formed from a crystalline micrporous MOF containing a 3-D system of channels with a pore diameter of 0.82 nm, resulting in hierarchically micro-and mesoporous MOFs.2. Triblock copolymers P123 and F127 were also chosen as supramolecular template to design and construct hierarchically micro-and mesoporous metal-organic framework. It was found that, mesopore system could not be tuned when only these triblock copolymers were selected as structure-directing agent, though mesostructured [Cu3(BTC)2(H2O)3]n were also be obtained. Only mesopores with diameters around 3.9 nm could be obtained by using these supramolecular template systems. However, by selecting 1,3,5-trimethylbenzene (TMB) as a swelling agent, mesopore sizes in these mesostructured MOF [Cu3(BTC)2(H2O)3]n could be tuned from 3.9 to 44 nm by changing P123/TMB molar ratio.3. Hydrogen adsorption at low pressure and catalysis properties of these hierarchically micro-and mesoporous MOFs have been studied. Low-pressure hydrogen storage results reveal that H2 gravimetric storage capacity of such the mesostructured MOF increases with an increase in BET surface area. In contrast, mesostructured [Cu3(BTC)2(H2O)3]n with larger pore size showed lower H2 uptake, which is its low surface area. Transformation of benzyl alcohol to benzaldehyde by using H2O2 as the oxidant was selected as a model reaction to evaluate catalysis property of these mesostructured MOFs. These hierarchically micro-and mesoporous MOFs exhibit excellent catalytic activity. The kinetic results showed that apparent reaction rate kobsd decreased slightly from 0.0388 to 0.0305 min-1 with an increase of mesopore diameter 0 to 4.9 nm. With increasing mesopore diameter from 4.9 to 14.9 nm, kobsd increased sharply to 0.619 min-1, suggesting that large mesopore size improve diffusion restraint effect of the substract and product in channels of the MOF. However, further increase in mesopore size led to a decrease in the reaction rate, which is due to low surface area of the MOF material. This result reveals that diffusion restraint effect of porous MOF can be eliminated or improved by designing and constructing such hierarchically micro-and mesoporous MOFs.